The present invention relates to apparatus for underwater depth measurement or contour mapping, and more particularly to apparatus for measurement of ship heave and for correction of underwater depth or contour mapping measurements therewith.
When a mobile vessel, such as a ship, floating on the surface of a body of water is used to determine the depth of that body of water, or to accomplish contour mapping of the bottom of that body of water, inaccuracies can result because of waves or swells on the surface of the water. These wave and swell effects can cause vertical displacement, hereinafter referred to as heave, of the ship or other vessel. Such contour plotting can be accomplished by transmitting a sonar, radar or other signal from the ship or other vessel vertically into the water to be bounced off the bottom of the body of water and returned to the ship for reception. Assuming constant velocity of the signal through the water, measurement of the depth of the body of water as the distance between the ship bottom and the water bottom at the surface location of the ship or other vessel can be accomplished by measuring the time delay between transmission and reception of the signal. However, if the ship or other vessel is vertically displaced above or below the normal surface of the body of water by wave or swell effects, this vertical displacement will be included in the measured depth. Accordingly, such depth measurements should be corrected to eliminate the effect of ship heave. Previous heave correction devices have employed a shipboard accelerometer to measure and produce a signal indicating ship vertical acceleration, which signal is then double integrated to produce displacement. However, such acceleration signals include noise, such as is caused by gravity, vertical coriolis, vertical vibration and electrical noise. This noise would, along with the portion of the acceleration signal attributable to heave, also be double integrated and included in the correction signal applied to the depth measurement, thereby causing error in the corrected depth measurement.